[R-sig-ME] modeling the effect of grass height on insect biomass

[Thierry Onkelinx]

Dear Joaquin,

Please keep the mailinglist in cc.

I'd rather think of the weeks being crossed with the fields than nested.
E.g. the effect of a week is probably similar on all fields. Then the model
looks like biomas~grass height +(1|Site/Field) + (1|Week). Note that you'll
need lme4 to fit that. crossed random effects are to straightforward in
nlme.

Random effects with as little as 4 or 5 levels might have unreliable
variance estimates. Keep that in mind when examining the modeloutput.

Best regards,

ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature and
Forest
team Biometrie & Kwaliteitszorg / team Biometrics & Quality Assurance
Kliniekstraat 25
1070 Anderlecht
Belgium

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to say
what the experiment died of. ~ Sir Ronald Aylmer Fisher
The plural of anecdote is not data. ~ Roger Brinner
The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of data.
~ John Tukey

2015-10-05 14:00 GMT+02:00 Joaquín Aldabe <joaquin.aldabe op gmail.com>:

> Thanks Thierry. Acutually, I collected each trap once a week, but decided
> to sum the the whole content (30 days). Should I conserve the weekly
> sampling? if so I guess the model could be insect biomas~grass height
> +(1|Site\Field\Week) ?
> Thanks again.
> Joaquín.
>
> 2015-10-05 9:40 GMT-02:00 Thierry Onkelinx <thierry.onkelinx op inbo.be>:
>
>> Dear Joaquin
>>
>> Your model seems reasonable, assuming that you measure the pitfalls only
>> once.
>>
>> Best regards,
>>
>> ir. Thierry Onkelinx
>> Instituut voor natuur- en bosonderzoek / Research Institute for Nature
>> and Forest
>> team Biometrie & Kwaliteitszorg / team Biometrics & Quality Assurance
>> Kliniekstraat 25
>> 1070 Anderlecht
>> Belgium
>>
>> To call in the statistician after the experiment is done may be no more
>> than asking him to perform a post-mortem examination: he may be able to say
>> what the experiment died of. ~ Sir Ronald Aylmer Fisher
>> The plural of anecdote is not data. ~ Roger Brinner
>> The combination of some data and an aching desire for an answer does not
>> ensure that a reasonable answer can be extracted from a given body of data.
>> ~ John Tukey
>>
>> 2015-10-05 13:22 GMT+02:00 Joaquín Aldabe <joaquin.aldabe op gmail.com>:
>>
>>> Dear all, I want to see the effect of grass height on insect biomass in a
>>> farm close to a coastal lagoon in Uruguay. For that I chose 5 pairs of
>>> fields. Each pair consist on a field with tall grass and the other with
>>> short grass. Each pair of fields are close to each other, in order to
>>> reduce the effect of soil type and other confounding variables. In each
>>> field I installed 6 pitfall traps, separated 100 m each. I collected
>>> insects for 30 days.
>>>
>>> Is the following modeling approach correct?: I called Site to each field
>>> pair (as they are geographically separated)
>>>
>>> model=lme(insect biomass~grass height + (1|Site\Field), data)
>>>
>>> Alternatively I thought of doing paired t-test..
>>>
>>> Thankyou very much for your help.
>>>
>>> Joaquín.
>>>
>>> --
>>> *Joaquín Aldabe*
>>>
>>> *Grupo Biodiversidad, Ambiente y Sociedad*
>>> Centro Universitario de la Región Este, Universidad de la República
>>> Ruta 15 (y Ruta 9), Km 28.500, Departamento de Rocha
>>>
>>> *Departamento de Conservación*
>>> Aves Uruguay
>>> BirdLife International
>>> Canelones 1164, Montevideo
>>>
>>> https://sites.google.com/site/joaquin.aldabe
>>> <https://sites.google.com/site/perfilprofesionaljoaquinaldabe>
>>>
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>>>
>>> _______________________________________________
>>> R-sig-mixed-models op r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
>>>
>>
>>
>
>
> --
> *Joaquín Aldabe*
>
> *Grupo Biodiversidad, Ambiente y Sociedad*
> Centro Universitario de la Región Este, Universidad de la República
> Ruta 15 (y Ruta 9), Km 28.500, Departamento de Rocha
>
> *Departamento de Conservación*
> Aves Uruguay
> BirdLife International
> Canelones 1164, Montevideo
>
> https://sites.google.com/site/joaquin.aldabe
> <https://sites.google.com/site/perfilprofesionaljoaquinaldabe>
>
>

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